The present invention relates to a multi-layered stretched resin film which is successfully exempt from being waved or curled due to vehicle contained in offset printing ink during offset printing, prevented from being delayed in the ink drying property, and thus has an excellent printability. Such multi-layered stretched resin film is advantageous when used as a material for map, book cover, wrapping paper, book paper, sticker sheet, poster paper, menu sheet, hanging screen, label, shopping bag, cardboard, brochure, illuminated sign board and so forth.
Synthetic paper has proposed and put into practical use in recent years in place of conventional natural pulp-made paper, where the synthetic paper comprises a base layer made of a biaxially stretched polypropylene film containing 5 to 40 wt % of an inorganic fine powder, and a paper-like layer stacked on the top and back planes thereof, which is made of a uniaxially stretched polypropylene film containing 8 to 65 wt % of an inorganic fine powder (U.S. Pat. No. 418,950, Japanese Examined Patent Publication Nos. 46-40794, 60-36173 and 62-35412).
Offset printing, so-called lithograph, is generally applied for printing on paper, polyethylene terephthalate film, polyamide film, coated paper and the like, since the technique can readily provide multi-color printing. Typical composition of a general-purpose, dry-type offset printing ink is shown below. 
The vehicle described in the above refers to a liquid component among various components composing printing ink, and is responsible for dispersing pigment to thereby keep fluidity of the ink; for ensuring smooth movement of the ink from an ink pot to a printing plane after being transferred among individual rollers, plate and blanket; and for fixing the pigment onto the printing plane through solidification. In response to recent strong requirement to shorten the drying time of the offset printing ink for more rapid printing, there is an increasing trend of using quick-drying ink having compounded therein a vehicle which mainly comprises a drying oil added with resin and mineral oil (high-boiling-point, petroleum-base solvent).
Printing with such quick-drying offset printing ink on the synthetic paper comprising a polyolefinic film or such film compounded with an inorganic fine powder, however, undesirably swells the polyolefin per se due to the vehicle contained in the quick-drying offset printing ink to thereby produce local surface waving or curling of the printed film, which has been an obstacle for the practical use. So that the conventional offset printing ink for polyolefinic film has been such that having a special formula without mineral oil at the sacrifice of the quick-drying property.
Such special offset printing ink for polyolefinic film, however, requires long drying time and has been only available in limited printing works and manufacturers, so that there has been a strong need for a new polyolefinic film on which the general-purpose offset printing ink of oxidation polymerization type (drying oil type) is available.
That is, in the general printing works, offset printing is practiced on pulp-base paper such as wood free paper and coated paper using generally-commercialized, quick-drying ink, so that printing on the polyolefinic film or synthetic paper requires temporary replacement of such ink with the special offset printing ink suitable for such non-absorbent material. Such replacement of the ink is, however, considerably time- and labor-consuming, so that the general printing works have not been positive about printing on the polyolefinic film or synthetic paper, which has been one factor obstructing the generalization of the polyolefinic film or synthetic paper in offset printing.
One approach has been made (Laid-Open Japanese Patent Publication No. 8-333466), in which a polyolefinic resin added with an amorphous resin is used as a part of the multi-layered film. Such technique is, however, still not successful in solving the problem of swelling of the film (referred to as xe2x80x9csolvent attackxe2x80x9d hereinafter) as a whole due to vehicle (in particular high-boiling-point, petroleum-base solvent such as mineral oil) contained in the offset printing ink.
It is therefore an object of the present invention to provide a multi-layered stretched resin film excellent in ink drying property, and less causative of surface waving or entire curling even if the film is subjected to offset printing using a general-purpose, quick-drying offset printing ink.
The present inventors found out after extensive investigations for solving the foregoing problems that adding an amorphous resin in a proper ratio to one layer composing a thermoplastic resin film, and stretching such film so as to lower the porosity of such layer successfully improve the solvent attack. It was also found that a proper blending of an inorganic fine powder or an organic filler in particular to the uppermost layer can improve the ink drying property and thus can provide a multi-layered stretched resin film excellent in the printing property.
The present invention is to provide a multi-layered stretched resin film comprising a base layer (A) containing 40 to 90 wt % of a polyolefinic resin and 10 to 60 wt % of an inorganic fine powder or an organic filler; and an amorphous resin-containing layer (B) provided on at least one side of such base layer (A), and containing 0 to 85 wt % of a polyolefinic resin and 15 to 100 wt % of an amorphous resin; in which amorphous resin-containing layer (B) has a porosity of 5% or below.
The amorphous resin used for the multi-layered stretched resin film of the present invention is preferably a cycloolefinic resin, and preferably has a glass transition point of 140xc2x0 C. or below. The polyolefinic resin contained in the amorphous resin-containing layer (B) is any one of propylene-base resin, ethylene-base resin and a mixture thereof. The amorphous resin-containing layer (B) preferably has a thickness of 1 to 100 xcexcm.
In the present invention, it is preferable that the multi-layered stretched resin film further comprises a surface layer (C) containing 30 to 85 wt % of a polyolefinic resin and 15 to 70 wt % of an inorganic fine powder or an organic filler, in which the surface layer (C) is provided on at least one side of a stack which comprises the base layer (A) and the amorphous resin-containing layer (B) provided on at least one side thereof. The surface layer (C) preferably has a thickness of 1 to 100 xcexcm. The grain size of the inorganic fine powder or the average dispersion grain size of the organic filler in the surface layer (C) is preferably within a range from 0.01 to 6 xcexcm, where the inorganic fine powder or the organic filler in the surface layer (C) preferably comprises calcium carbonate, in particular surface-modified calcium carbonate. The surface layer (C) preferably has a smoothness in compliance with JIS P-8119 of 20,000 sec or below.
In the present invention, layer constitution involving the base layer (A) and the amorphous resin-containing layer (B) and/or surface layer(C) are preferably such that being expressed as (C)/(B)/(A)/(B)/(C), (C)/(B)/(A)/(B), (C)/(B)/(A)/(C), (C)/(B)/(A), (B)/(A)/(B), (B)/(A)/(C) and (B)/(A).
It is preferable that the multi-layered stretched resin film of the present invention further comprises an intermediate layer (D) comprising a polypropylene-base resin containing 8 to 55 wt % of an inorganic fine powder, in which the intermediate layer (D) is provided between the base layer (A) and the amorphous resin-containing layer (B) or the surface layer (C). The intermediate layer (D) preferably contains any one low-melting-point resin selected from the group comprising propylene-base copolymer, high-density polyethylene, polystyrene or ethylene-vinyl acetate copolymer.
The multi-layered stretched resin film of the present invention preferably has an opacity in compliance with JIS P-8138 of 70% or above, and preferably has on the outermost layer thereof a pigment coated layer.
It should now be noted that, in this specification, any notation using a word xe2x80x9ctoxe2x80x9d indicates a range defined by values placed before and after such word, where both ends of such range are included as minimum and maximum values.